Polymer mixtures for crosslinked fluids

ABSTRACT

Treatment fluids comprising gelling agents that comprise crosslinkable polymers and certain biopolymers, and methods of use in subterranean operations, are provided. In one embodiment, the present invention provides a treatment fluid comprising: an aqueous base fluid; a crosslinking agent; and a gelling agent comprising a polymer that is a crosslinkable polymer, and a polymer that is a biopolymer wherein a molecule of the biopolymer (1) consists only of glucose, or (2) has a backbone comprising one or more units that comprise at least (a) one glucose unit and (b) one linear or cyclic pyranose-type monosaccharide unit, wherein (a) and (b) have different molecular structures.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present invention is related to co-pending U.S. application Ser. No.______ , Attorney Docket No. HES 2004-IP-014518U1 entitled “PolymerMixtures for Crosslinked Fluids,” filed concurrently herewith, theentire disclosure of which is incorporated herein by reference.

BACKGROUND

The present invention relates to methods and compositions for treatingsubterranean formations. More particularly, the present inventionrelates to treatment fluids comprising gelling agents that comprisecrosslinkable polymers and certain biopolymers, and methods of use insubterranean operations.

Treatment fluids may be used in a variety of subterranean treatments,including, but not limited to, stimulation treatments and sand controltreatments. As used herein, the term “treatment,” or “treating,” refersto any subterranean operation that uses a fluid in conjunction with adesired function and/or for a desired purpose. The term “treatment,” or“treating,” does not imply any particular action by the fluid or anyparticular component thereof.

One common production stimulation operation that employs a treatmentfluid is hydraulic fracturing. Hydraulic fracturing operations generallyinvolve pumping a treatment fluid (e.g., a fracturing fluid) into a wellbore that penetrates a subterranean formation at a sufficient hydraulicpressure to create or enhance one or more cracks, or “fractures,” in thesubterranean formation. “Enhancing” one or more fractures in asubterranean formation, as that term is used herein, is defined toinclude the extension or enlargement of one or more natural orpreviously created fractures in the subterranean formation. Thefracturing fluid may comprise particulates, often referred to as“proppant particulates,” that are deposited in the fractures. Theproppant particulates function, inter alia, to prevent the fracturesfrom fully closing upon the release of hydraulic pressure, formingconductive channels through which fluids may flow to the well bore. Onceat least one fracture is created and the proppant particulates aresubstantially in place, the fracturing fluid may be “broken” (i.e., theviscosity of the fluid is reduced), and the fracturing fluid may berecovered from the formation.

Treatment fluids are also utilized in sand control treatments, such asgravel packing. In gravel-packing treatments, a treatment fluid suspendsparticulates (commonly referred to as “gravel particulates”) to bedeposited in a desired area in a well bore, e.g., near unconsolidated orweakly consolidated formation zones, to form a gravel pack to enhancesand control. One common type of gravel-packing operation involvesplacing a sand control screen in the well bore and packing the annulusbetween the screen and the well bore with the gravel particulates of aspecific size designed to prevent the passage of formation sand. Thegravel particulates act, inter alia, to prevent the formationparticulates from occluding the screen or migrating with the producedhydrocarbons, and the screen acts, inter alia, to prevent theparticulates from entering the production tubing. Once the gravel packis substantially in place, the viscosity of the treatment fluid may bereduced to allow it to be recovered. In some situations, fracturing andgravel-packing treatments are combined into a single treatment (commonlyreferred to as “frac pack” operations). In such “frac pack” operations,the treatments are generally completed with a gravel pack screenassembly in place with the hydraulic fracturing treatment being pumpedthrough the annular space between the casing and screen. In thissituation, the hydraulic fracturing treatment ends in a screen-outcondition, creating an annular gravel pack between the screen andcasing. In other cases, the fracturing treatment may be performed priorto installing the screen and placing a gravel pack.

Maintaining sufficient viscosity in these treatment fluids is importantfor a number of reasons. Maintaining sufficient viscosity is importantin fracturing and sand control treatments for particulate transportand/or to create or enhance fracture width. Also, maintaining sufficientviscosity may be important to control and/or reduce fluid-loss into theformation. At the same time, while maintaining sufficient viscosity ofthe treatment fluid often is desirable, it may also be desirable tomaintain the viscosity of the treatment fluid in such a way that theviscosity also may be easily reduced at a particular time, inter alia,for subsequent recovery of the fluid from the formation.

To provide the desired viscosity, polymeric gelling agents commonly areadded to the treatment fluids. The term “gelling agent” is definedherein to include any substance that is capable of increasing theviscosity of a fluid, for example, by forming a gel. Examples ofcommonly used polymeric gelling agents include, but are not limited to,guar gums and derivatives thereof, cellulose derivatives, biopolymers,and the like. However, the use of a single gelling agent willnecessarily limit the performance of the fluid to those properties thatthe single gelling agent used can impart to the treatment fluid, to theexclusion of other properties that other gelling agents might impart tothe treatment fluid.

To further increase the viscosity of a treatment fluid, often thegelling agent is crosslinked with the use of a crosslinking agent.Conventional crosslinking agents usually comprise a metal ion thatinteracts with at least two gelling agent molecules to form a crosslinkbetween them, thereby forming a “crosslinked gelling agent.” Treatmentfluids comprising crosslinked gelling agents also may exhibit elastic orviscoelastic properties, wherein the crosslinks between gelling agentmolecules may be broken and reformed, allowing the viscosity of thefluid to vary with certain conditions such as temperature, pH, and thelike. However, the use of such crosslinking agents may be problematic.For example, in some instances, the gelling agent molecules may“over-crosslink” in the presence of high concentrations of crosslinkingagent, yielding a treatment fluid that is over-viscosified, difficult tobreak, exhibits syneresis (i.e., separation of liquid in a gel), or hasother undesirable rheological properties.

Treatment fluids comprising mixtures of xanthan and a guar gum are knownin the art. When compared to treatment fluids that contain a singlepolymer gelling agent, treatments fluids containing a mixture of xanthanand a guar gum may improve proppant transport capabilities of thetreatment fluid. However, mixtures comprising xanthan and a guar gumhave been found to be unstable at higher temperature conditions (e.g.,greater than about 150° F.) in certain subterranean formations and wellbores where treatment fluids comprising these mixtures may be useful.

SUMMARY

The present invention relates to methods and compositions for treatingsubterranean formations. More particularly, the present inventionrelates to treatment fluids comprising gelling agents that comprisecrosslinkable polymers and certain biopolymers, and methods of use insubterranean operations.

In one embodiment, the present invention provides a treatment fluidcomprising: an aqueous base fluid; a crosslinking agent; and a gellingagent comprising a polymer that is a crosslinkable polymer, and apolymer that is a biopolymer wherein a molecule of the biopolymer (1)consists only of glucose, or (2) has a backbone comprising one or moreunits that comprise at least (a) one glucose unit and (b) one linear orcyclic pyranose-type monosaccharide unit, wherein (a) and (b) havedifferent molecular structures.

In another embodiment, the present invention provides a treatment fluidcomprising: an aqueous base fluid; a crosslinking agent; and a gellingagent comprising a polymer that is a crosslinkable polymer, and diutan.

In another embodiment, the present invention provides a gelling agentcomprising: a polymer that is a crosslinkable polymer; a crosslinkingagent; and a polymer that is biopolymer wherein a molecule of thebiopolymer (1) consists only of glucose, or (2) has a backbonecomprising one or more units that comprise at least (a) one glucose unitand (b) one linear or cyclic pyranose-type monosaccharide unit, wherein(a) and (b) have different molecular structures.

The features and advantages of the present invention will be readilyapparent to those skilled in the art upon a reading of the descriptionof the preferred embodiments that follows.

BRIEF DESCRIPTION OF THE DRAWINGS

These drawings illustrate certain aspects of some of the embodiments ofthe present invention and should not be used to limit or define theinvention.

FIGS. 1 a, 1 b, 1 c illustrate data regarding the viscoelasticproperties of various treatment fluids, including certain embodiments ofthe treatment fluids of the present invention.

FIG. 2 illustrates data regarding other viscoelastic properties ofvarious treatment fluids, including certain embodiments of the treatmentfluids of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention relates to methods and compositions for treatingsubterranean formations. More particularly, the present inventionrelates to treatment fluids comprising gelling agents that comprisecrosslinkable polymers and certain biopolymers, and methods of use insubterranean operations.

The treatment fluids of the present invention generally comprise anaqueous base fluid, a gelling agent comprising certain biopolymers and acrosslinkable polymer, and a crosslinking agent. As used herein, theterm “crosslinkable polymer” is defined to mean any polymeric materialcomprising at least two molecules that are capable of forming acrosslink in the presence of a suitable crosslinking agent. Among otherthings, the treatment fluids of the present invention may exhibitenhanced viscoelastic properties, such as low shear viscosity, anddecreased levels of syneresis over other treatment fluids known in theart. These viscoelastic properties may enable, for example, slowerproppant settling, improved proppant transport, and extended relaxationtimes in some treatment fluids the present invention.

The aqueous base fluid used in the treatment fluids of the presentinvention may comprise fresh water, saltwater (e.g., water containingone or more salts dissolved therein), brine (e.g., saturated saltwater),seawater, or combinations thereof. Generally, the water may be from anysource, provided that it does not contain components that mightadversely affect the stability and/or performance of the treatmentfluids of the present invention. In certain embodiments, the density ofthe aqueous base fluid can be increased, among other purposes, toprovide additional particle transport and suspension in the treatmentfluids of the present invention. In certain embodiments, the pH of theaqueous base fluid may be adjusted (e.g., by a buffer or other pHadjusting agent), among other purposes, to facilitate hydration of thebiopolymer, to activate a crosslinking agent, and/or to reduce theviscosity of the treatment fluid (e.g., activate a breaker, deactivate acrosslinking agent). In these embodiments, the pH may be adjusted to aspecific level, which may depend on, among other factors, the types ofbiopolymers, crosslinking agents, and/or breakers included in thetreatment fluid. One of ordinary skill in the art, with the benefit ofthis disclosure, will recognize when such density and/or pH adjustmentsare appropriate.

The gelling agent included in the treatment fluids of the presentinvention comprises certain biopolymers. The biopolymers utilized in thepresent invention have structures wherein a molecule of the biopolymer(1) consists only of glucose, or (2) has a backbone comprising one ormore units that comprise at least (a) one glucose unit and (b) onelinear or cyclic pyranose-type monosaccharide unit, wherein (a) and (b)have different molecular structures. The term “backbone,” as usedherein, refers to the longest sequence of units in the biopolymermolecule. Pyranose-type monosaccharides are generally characterized ashaving 5 carbon atoms and an oxygen atom in a ring of 6 atoms. Incontrast to biopolymers that may be included in the treatment fluidsalready known in the art, the biopolymers utilized in the presentinvention do not comprise xanthan. Examples of suitable biopolymersinclude, but are not limited to, diutan, scleroglucan, succinoglycan,and combinations thereof and derivatives thereof. As used herein, theterm “derivative” is defined to include any compound that is made fromthe base compound, for example, by replacing one atom in the basecompound with another atom or group of atoms, ionizing one of the listedcompounds, or creating a salt of one of the listed compounds. Thebiopolymers utilized in the present invention may or may not becrosslinked by one or more crosslinking agents. In certain embodimentsof the present invention, the biopolymer may be provided in aconcentrated aqueous solution prior to its combination with the othercomponents necessary to form a treatment fluid of the present invention.In certain embodiments of the present invention, the biopolymer may beprovided in a solution that comprises other components of the treatmentfluid and/or the gelling agent (e.g., the crosslinkable polymer).

Generally, the biopolymer may be present in the treatment fluids of thepresent invention in an amount sufficient to provide the desiredviscosity. In certain embodiments, the biopolymer may be present in anamount in the range of from about 0.01% to about 3% by weight of thetreatment fluid (“bwof”). In certain exemplary embodiments, thebiopolymer may be present in an amount in the range of from about 0.1%to about 1% bwof. In certain embodiments, the biopolymers may be presentin the treatment fluids of the present invention in abiopolymer-to-crosslinkable polymer ratio in the range of from about0.05:1 to about 1:1. In certain embodiments, the biopolymers may bepresent in the treatment fluids of the present invention in abiopolymer-to-crosslinkable polymer ratio of about 0.1:1. Thebiopolymer-to-crosslinkable polymer ratio is dependent on a variety offactors, such as the desired viscosity and/or elasticity, the particularapplication, downhole conditions, water quality, and the like, whichwill be recognizable by a person skilled in the art.

The gelling agent included in the treatment fluids of the presentinvention may comprise any suitable crosslinkable polymer, including,but not limited to, galactomannan gums, cellulose derivatives,combinations thereof, derivatives thereof, and the like. Galactomannangums are generally characterized as having a linear mannan backbone withvarious amounts of galactose units attached thereto. Examples ofsuitable galactomannan gums include, but are not limited to, gum arabic,gum ghatti, gum karaya, tamarind gum, tragacanth gum, guar gum, locustbean gum, combinations thereof, derivatives thereof, and the like. Othersuitable gums include, but are not limited to, hydroxyethylguar,hydroxypropylguar, carboxymethylguar, carboxymethylhydroxyethylguar andcarboxymethylhydroxypropylguar. Examples of suitable cellulosederivatives include hydroxyethyl cellulose, carboxyethylcellulose,carboxymethylcellulose, and carboxymethylhydroxyethylcellulose;derivatives thereof, and combinations thereof. The crosslinkablepolymers included in the treatment fluids of the present invention maybe naturally-occurring, synthetic, or a combination thereof. Thecrosslinkable polymers may comprise hydratable polymers that contain oneor more functional groups such as hydroxyl, cis-hydroxyl, carboxyl,sulfate, sulfonate, phosphate, phosphonate, amino, or amide groups. Incertain embodiments, the crosslinkable polymers may be at leastpartially crosslinked, wherein at least a portion of the molecules ofthe crosslinkable polymers are crosslinked by a reaction comprising acrosslinking agent.

The crosslinkable polymers should be present in the treatment fluids ofthe present invention in an amount sufficient to provide the desiredviscosity of the treatment fluid. In certain embodiments, thecrosslinkable polymers may be present in an amount in the range of fromabout 0.05% to about 3% bwof. In certain embodiments, the crosslinkablepolymers may be present in an amount in the range of from about 0.1% toabout 1% bwof. In certain embodiments, the crosslinkable polymers may bepresent in a crosslinkable polymer-to-biopolymer ratio in the range offrom about 1:1 to about 20:1. In certain embodiments, the crosslinkablepolymers may be present in a crosslinkable polymer-to-biopolymer ratioof about 9:1. The crosslinkable polymer-to-biopolymer ratio is dependenton a variety of factors, such as the desired viscosity and/orelasticity, the particular application, downhole conditions, waterquality, and the like, which will be recognizable by a person skilled inthe art. In certain embodiments of the present invention, thecrosslinkable polymers and/or biopolymers may be provided in aconcentrated aqueous solution prior to its combination with the othercomponents necessary to form a treatment fluid of the present invention.In certain embodiments of the present invention, the crosslinkablepolymers may be provided in a solution that comprises other componentsof the treatment fluid and/or the gelling agent (e.g., the biopolymer).

Crosslinking agents are generally included in the treatment fluids ofthe present invention to crosslink at least a portion of the moleculesof the crosslinkable polymers to form a crosslinked polymer. The term“crosslinking agent” is defined herein to include any molecule, atom, orion that is capable of forming one or more crosslinks between moleculesof the crosslinkable polymer and/or between one or more atoms in asingle molecule of the crosslinkable polymer. The crosslinking agent inthe treatment fluids of the present invention may comprise a metal ionthat is capable of crosslinking at least two molecules of thecrosslinkable polymer. Examples of suitable crosslinking agents include,but are not limited to, borate ions, zirconium IV ions, titanium IVions, aluminum ions, antimony ions, chromium ions, iron ions, copperions, and zinc ions. These ions may be provided by providing anycompound that is capable of producing one or more of these ions;examples of such compounds include, but are not limited to, boric acid,disodium octaborate tetrahydrate, sodium diborate, pentaborates,ulexite, colemanite, zirconium lactate, zirconium lactatetriethanolamine, zirconium carbonate, zirconium acetylacetonate,zirconium malate, zirconium citrate, zirconium diisopropylamine lactate,titanium lactate, titanium malate, titanium citrate, titanium ammoniumlactate, titanium triethanolamine, and titanium acetylacetonate,aluminum lactate, aluminum citrate, antimony compounds, chromiumcompounds, iron compounds, copper compounds, zinc compounds, andcombinations thereof. An example of a suitable commercially availablecompound capable of providing metal ions is “CL-24TM” crosslinkeravailable from Halliburton Energy Services, Inc., Duncan, Okla. Incertain embodiments of the present invention, the crosslinking agent maybe present in a crosslinked polymer, wherein at least a portion of themolecules of the crosslinkable polymer are crosslinked by thecrosslinking agent.

In some embodiments, the crosslinking agent may comprise a delayedcrosslinking agent, which may be formulated to form crosslinks betweenpolymer molecules after a certain time or under certain conditions(e.g., temperature, pH, etc.). In some embodiments, the treatment fluidmay comprise a crosslinking delaying agent, such as a polysaccharidecrosslinking delaying agents derived from guar, guar derivatives, orcellulose derivatives. The crosslinking delaying agent may be includedin the treatment fluid, inter alia, to delay crosslinking of thecrosslinkable polymers until desired. One of ordinary skill in the art,with the benefit of this disclosure, will know the appropriate amount ofthe crosslinking delaying agent to include in the treatment fluids for adesired application.

Suitable crosslinking agents may be present in the treatment fluids ofthe present invention in an amount sufficient to provide, inter alia,the desired degree of crosslinking between molecules of thecrosslinkable polymers. In certain embodiments, the crosslinking agentmay be present in the treatment fluids of the present invention in anamount in the range of from about 10 parts per million (“ppm”) to about500 ppm by weight of the treatment fluid. In certain exemplaryembodiments, the crosslinking agent may be present in the treatmentfluids of the present invention in an amount in the range of from about75 ppm to about 200 ppm by weight of the treatment fluid. One ofordinary skill in the art, with the benefit of this disclosure, willrecognize the appropriate type and amount of crosslinking agent toinclude in a treatment fluid of the present invention based on, amongother things, the temperature conditions of a particular application,the type of crosslinkable polymers used, the molecular weight of thecrosslinkable polymers, and/or the pH of the treatment fluid.

The treatment fluids of the present invention optionally may compriseparticulates, such as proppant particulates or gravel particulates.Particulates suitable for use in the present invention may comprise anymaterial suitable for use in subterranean operations. Suitable materialsfor these particulates include, but are not limited to, sand, bauxite,ceramic materials, glass materials, polymer materials, Teflon®materials, nut shell pieces, cured resinous particulates comprising nutshell pieces, seed shell pieces, cured resinous particulates comprisingseed shell pieces, fruit pit pieces, cured resinous particulatescomprising fruit pit pieces, wood, composite particulates, andcombinations thereof. Suitable composite particulates may comprise abinder and a filler material wherein suitable filler materials includesilica, alumina, fumed carbon, carbon black, graphite, mica, titaniumdioxide, meta-silicate, calcium silicate, kaolin, talc, zirconia, boron,fly ash, hollow glass microspheres, solid glass, and combinationsthereof. The particulate size generally may range from about 2 mesh toabout 400 mesh on the U.S. Sieve Series; however, in certaincircumstances, other sizes may be desired and will be entirely suitablefor practice of the present invention. In particular embodiments,preferred particulates size distribution ranges are one or more of 6/12,8/16, 12/20, 16/30, 20/40, 30/50, 40/60, 40/70, or 50/70 mesh. It shouldbe understood that the term “particulate,” as used in this disclosure,includes all known shapes of materials, including substantiallyspherical materials, fibrous materials, polygonal materials (such ascubic materials), and mixtures thereof. Moreover, fibrous materials,that may or may not be used to bear the pressure of a closed fracture,may be included in certain embodiments of the present invention. Incertain embodiments, the particulates included in the treatment fluidsof the present invention may be coated with any suitable resin ortackifying agent known to those of ordinary skill in the art. In certainembodiments, the particulates may be present in the treatment fluids ofthe present invention in an amount in the range of from about 0.5 poundsper gallon (“ppg”) to about 30 ppg by volume of the treatment fluid.

The gelled or gelled and cross-linked treatment fluids may also includeinternal delayed gel breakers such as enzyme, oxidizing, acid buffer, ortemperature-activated gel breakers. The gel breakers may cause theviscous treatment fluids to revert to thin fluids that can be producedback to the surface after they have been used to place proppantparticles in subterranean fractures. In certain embodiments, the gelbreaker used may be present in the treatment fluid in an amount in therange of from about 0.0001% to about 10% by weight of the gelling agent.

The treatment fluids of the present invention optionally may include oneor more of a variety of well-known additives, such as gel stabilizers,fluid loss control additives, acids, corrosion inhibitors, catalysts,clay stabilizers, biocides, bactericides, friction reducers, gas,surfactants, solubilizers, pH adjusting agents, and the like. Forexample, in some embodiments, it may be desired to foam a treatmentfluid of the present invention using a gas, such as air, nitrogen, orcarbon dioxide. Those of ordinary skill in the art, with the benefit ofthis disclosure, will be able to determine the appropriate additives fora particular application.

The treatment fluids of the present invention may be prepared by anymethod suitable for a given application. For example, certain componentsof the treatment fluid of the present invention (e.g., crosslinkablepolymers, biopolymers, etc.) may be provided in a pre-blended powder,which may be combined with the aqueous base fluid at a subsequent time.In preparing the treatment fluids of the present invention, the pH ofthe aqueous base fluid may be adjusted, among other purposes, tofacilitate the hydration of the gelling agent. The pH range in which thegelling agent will readily hydrate may depend upon a variety of factors(e.g., the components of the gelling agent, etc.) that will berecognized by one skilled in the art. This adjustment of pH may occurprior to, during, or subsequent to the addition of the gelling agentand/or other components of the treatment fluids of the presentinvention. For example, the treatment fluids of the present inventionmay comprise an ester that releases an acid once placed downhole that iscapable of, inter alia, reducing the pH and/or viscosity of thetreatment fluid. After the preblended powders and the aqueous base fluidhave been combined crosslinking agents and other suitable additives maybe added prior to introduction into the well bore. Those of ordinaryskill in the art, with the benefit of this disclosure will be able todetermine other suitable methods for the preparation of the treatmentsfluids of the present invention.

The methods of the present invention may be employed in any subterraneantreatment where a viscoelastic treatment fluid may be used. Suitablesubterranean treatments may include, but are not limited to, fracturingtreatments, sand control treatments (e.g., gravel packing), and othersuitable treatments where a treatment fluid of the present invention maybe suitable. In one embodiment, the present invention provides a methodof treating a portion of a subterranean formation comprising: providinga treatment fluid that comprises an aqueous base fluid, crosslinkingagent, and a gelling agent comprising a first polymer that is acrosslinkable polymer and a second polymer that is a biopolymer whereina molecule of the biopolymer (1) consists only of glucose, or (2) has abackbone comprising one or more units that comprise at least (a) oneglucose unit and (b) one linear or cyclic pyranose-type monosaccharideunit, wherein (a) and (b) have different molecular structures; andintroducing the treatment fluid into a well bore penetrating thesubterranean formation. Subsequent to the introduction of the treatmentfluid into the well bore, the viscosity of the treatment fluid may bereduced at a desired time, and the reduced viscosity treatment fluid maybe recovered and/or produced back through the well bore.

To facilitate a better understanding of the present invention, thefollowing examples of specific embodiments are given. In no way shouldthe following examples be read to limit or define the entire scope ofthe invention.

EXAMPLES

As used in the Examples below, the unit “ppt” refers to pounds of dryingredient per 1000 gallons of fluid.

Example 1

Samples of three treatment fluids of the following compositions wereprepared in 400 mL jars: Sample Fluid 1 contained 45 ppt guar in asolution of 2% KCI by weight in tap water obtained in Duncan, Okla.;Sample Fluid 2 contained 40.5 ppt guar and 4.5 ppt xanthan in a solutionof 2% KCl by weight in tap water obtained in Duncan, Oklahoma; andSample Fluid 3 (a sample of a treatment fluid of the present invention)contained 40.5 ppt guar and 4.5 ppt diutan in a solution of 2% KCl byweight in tap water obtained in Duncan, Okla. 20/40 Brady sand was addedto each sample in an amount of 3 pounds per gallon of fluid (“ppg”), andthen each sample was heated in a water bath to 77° F. Once the samplesreached that temperature, a predetermined amount (listed in Table 1below) of a borate crosslinking agent was blended into each sample usinga Waring blender for 30 seconds. Afterwards, the samples were returnedto the water bath. Table 1 below lists the settle time (i.e., time atwhich 3 ppg sand completely settled in the jar) for each of the threesamples.

A small-amplitude oscillatory shear (“SAOS”) frequency sweep test wasalso performed on each of the sample fluids using a cylindrical couettefixture on a Stresstech rheometer (available from RheologicaInstruments), both before and after the fluid samples were crosslinked.In this test, a sinusoidal shear strain is applied to the sample in theform of γ=γ₀ sinωt, where γ₀ is the strain amplitude and ω is theoscillation frequency. The shear stress response is σ=G′sin ωt +G″cosωt,where G′ is the storage modulus in phase with the applied shear strainand G″ is the loss modulus out of phase with the applied shear strain(or in phase with the applied shear-strain rate).

A plot of frequency versus storage modulus and loss modulus from theSAOS frequency sweep for the base and crosslinked fluids from each ofthe three samples is provided in FIGS. 1 a, 1 b and 1 c, respectively.Table 1 below also lists the crossover frequency (∫_(c)) (i.e., thefrequency where G′=G″, the point where the elastic response of the fluidat high frequencies is separated from the viscous response at lowfrequencies, indicated on each figure) for the crosslinked fluid fromeach of the three samples. Lower crossover frequency values generallytranslate into higher relaxation times for a fluid, which indicatesimproved static suspension properties of the fluid. TABLE 1 Amount ofborate crosslinking agent Settle Time (gallons per 1000 gal (hours)Crossover Frequency fluid (“gpt”)) (approx.) (ω_(c)) (rad/s) Sample 1 80.16 Fluid 1 Sample 1 (suspended)¹ — Fluid 2 Sample 1 12 0.11 Fluid 3¹The sand in this Sample Fluid remained suspended and did not settle.

Example 2

Samples of three treatment fluids of the compositions described inExample 1 above were prepared in 400 mL jars. 20/40 Brady sand was addedto each sample in an amount of 3 ppg, and then each sample was heated ina water bath to 150° F. Once the samples reached that temperature, apredetermined amount (listed in Table 2 below) of a borate crosslinkingagent was blended into each sample using a Waring blender for 30seconds. Afterwards, the samples were returned to the water bath. AnSAOS frequency sweep test as described in Example 1 above was alsoperformed on each of the samples. Table 2 below lists the settle time(i.e., time at which 3 ppg sand completely settled in the jar) and thecrossover frequency (ω) for each of the three samples. TABLE 2 Amount ofborate Settle Time crosslinking agent (hours) Crossover Frequency (gpt)(approx.) (ω_(c)) (rad/s) Sample 3.75 1 0.46 Fluid 1 Sample 4.25 1 0.90Fluid 2 Sample 4.75 2 0.20 Fluid 3

Example 3

Samples of three treatment fluids of the compositions described inExample 1 above were prepared in 400 mL jars. 20/40 Brady sand was addedto each sample fluid in an amount of 3 ppg, and then each sample washeated in a water bath to 150° F. Once the samples reached thattemperature, a borate crosslinking agent was blended into each sample inan amount of 5 gpt using a Waring blender for 30 seconds. Afterwards,the samples were returned to the water bath. The amount of solventexpelled from each fluid sample was recorded every 10 minutes over aperiod of one hour. These observations are listed in Table 3 below, anda plot of time (min) versus total solvent expelled (vol %) is providedin FIG. 2. TABLE 3 Total solvent expelled (vol %) Time (min) SampleFluid 1 Sample Fluid 2 Sample Fluid 3 10 10 0 0 20 28 10 0 30 48 24 4 4052 28 4 50 56 32 6 60 60 32 8

Thus Examples 1-3 illustrate that the treatment fluids of the presentinvention may exhibit enhanced rheological properties.

Therefore, the present invention is well adapted to attain the ends andadvantages mentioned as well as those that are inherent therein. Theparticular embodiments disclosed above are illustrative only, as thepresent invention may be modified and practiced in different butequivalent manners apparent to those skilled in the art having thebenefit of the teachings herein. While numerous changes may be made bythose skilled in the art, such changes are encompassed within the spiritof this invention as defined by the appended claims. Furthermore, nolimitations are intended to the details of construction or design hereinshown, other than as described in the claims below. It is thereforeevident that the particular illustrative embodiments disclosed above maybe altered or modified and all such variations are considered within thescope and spirit of the present invention. In particular, every range ofvalues (e.g., “from about a to about b,” or, equivalently, “fromapproximately a to b,” or, equivalently, “from approximately a-b”)disclosed herein is to be understood as referring to the power set (theset of all subsets) of the respective range of values. The terms in theclaims have their plain, ordinary meaning unless otherwise explicitlyand clearly defined by the patentee.

1. A treatment fluid comprising: an aqueous base fluid; a crosslinkingagent; and a gelling agent comprising a polymer that is a crosslinkablepolymer, and a polymer that is a biopolymer wherein a molecule of thebiopolymer (1) consists only of glucose, or (2) has a backbonecomprising one or more units that comprise at least (a) one glucose unitand (b) one linear or cyclic pyranose-type monosaccharide unit, wherein(a) and (b) have different molecular structures.
 2. The treatment fluidof claim 1 wherein the biopolymer is selected from the group consistingof diutan, scleroglucan, succinoglycan, derivatives thereof, andcombinations thereof.
 3. The treatment fluid of claim 1 wherein thecrosslinkable polymer is selected from the group consisting of gumarabic, gum ghatti, gum karaya, tamarind gum, tragacanth gum, guar gum,locust bean gum, cellulose, derivatives thereof, and combinationsthereof.
 4. The treatment fluid of claim 1 wherein at least a portion ofthe molecules of the crosslinkable polymer are crosslinked by a reactioncomprising the crosslinking agent.
 5. The treatment fluid of claim 1wherein the crosslinking agent is selected from the group consisting ofborate ions, zirconium IV ions, titanium IV ions, aluminum ions,antimony ions, chromium ions, iron ions, copper ions, zinc ions, andcombinations thereof.
 6. The treatment fluid of claim 1 wherein thecrosslinking agent comprises a delayed crosslinking agent.
 7. Thetreatment fluid of claim 1 further comprising at least one elementselected from the group consisting of gel breakers, gel stabilizers,fluid loss control additives, acids, corrosion inhibitors, catalysts,clay stabilizers, biocides, bactericides, friction reducers, gases,surfactants, crosslinking delaying agents, solubilizers, particulates,pH adjusting agents, derivatives thereof, and combinations thereof.
 8. Atreatment fluid comprising: an aqueous base fluid; a crosslinking agent;and a gelling agent comprising a polymer that is a crosslinkablepolymer, and diutan.
 9. The treatment fluid of claim 8 wherein thecrosslinkable polymer is selected from the group consisting of gumarabic, gum ghatti, gum karaya, tamarind gum, tragacanth gum, guar gum,locust bean gum, cellulose, derivatives thereof, and combinationsthereof.
 10. The treatment fluid of claim 8 wherein at least a portionof the molecules of the crosslinkable polymer are crosslinked by areaction comprising the crosslinking agent.
 11. The treatment fluid ofclaim 8 wherein the crosslinking agent is selected from the groupconsisting of borate ions, zirconium IV ions, titanium IV ions, aluminumions, antimony ions, chromium ions, iron ions, copper ions, zinc ions,and combinations thereof.
 12. The treatment fluid of claim 8 wherein thecrosslinking agent comprises a delayed crosslinking agent.
 13. Thetreatment fluid of claim 8 further comprising at least one elementselected from the group consisting of gel breakers, gel stabilizers,fluid loss control additives, acids, corrosion inhibitors, catalysts,clay stabilizers, biocides, bactericides, friction reducers, gases,surfactants, crosslinking delaying agents, solubilizers, particulates,pH adjusting agents, derivatives thereof, and combinations thereof. 14.A gelling agent comprising: a polymer that is a crosslinkable polymer; acrosslinking agent; and a polymer that is biopolymer wherein a moleculeof the biopolymer (1) consists only of glucose, or (2) has a backbonecomprising one or more units that comprise at least (a) one glucose unitand (b) one linear or cyclic pyranose-type monosaccharide unit, wherein(a) and (b) have different molecular structures.
 15. The gelling agentof claim 14 wherein the biopolymer is selected from the group consistingof diutan, scleroglucan, succinoglycan, derivatives thereof, andcombinations thereof.
 16. The gelling agent of claim 14 wherein thebiopolymer is diutan.
 17. The gelling agent of claim 14 wherein thecrosslinkable polymer is selected from the group consisting of gumarabic, gum ghatti, gum karaya, tamarind gum, tragacanth gum, guar gum,locust bean gum, cellulose, derivatives thereof, and combinationsthereof.
 18. The gelling agent of claim 14 wherein at least a portion ofthe molecules of the crosslinkable polymer are crosslinked by a reactioncomprising the crosslinking agent.
 19. The gelling agent of claim 14wherein the crosslinking agent is selected from the group consisting ofborate ions, zirconium IV ions, titanium IV ions, aluminum ions,antimony ions, chromium ions, iron ions, copper ions, zinc ions, andcombinations thereof.
 20. The gelling agent of claim 14 wherein thecrosslinking agent comprises a delayed crosslinking agent.